Epoxy resins have the ability to remain liquid for long periods of time and then be converted upon addition of hardening agents to produce products such as glass reinforced pipe and molded fittings. This hardening is obtained by the addition of active curing or hardening agents. Two general types of reactions are involved in producing end products, that is, cross linking and catalytic polymerization through the epoxy group.
In applications involving premix, the ability to store mixtures of the epoxy resins and curing agents for extended periods of time and then cure the mixture at elevated temperatures to produce products having good chemical and physical properties is desirable. Conventional epoxy curing agents have disadvantages that limit their industrial use. For example, the Lewis acids, such as boron trifluoride monoethanol amine complex, react catalytically with epoxy resins and impart excellent latency to the formulation. However, molded parts cured with this material generally have lower corrosion resistance and less physical toughness than products cured with the preferred amine curing agents.
Aliphatic amines set up rapidly, necessitating mixing the components just before use and rapid utilization of the material before hardening. The time requirements make it economically unfeasible to prepare and mold epoxy resin premix formulations in which an aliphatic amine is employed as the hardener.
Aromatic amines cure epoxy resins at elevated temperatures to tough chemically resistant products. With respect to latency, aromatic amines are between Lewis acids and aliphatic amines. When aromatic amines are employed to cure epoxy resins, heat is required to obtain useful finished products. However, when aromatic amines are employed in curing epoxy resins, semi-thermal plastic or B-staged resins are rapidly formed during the early stages of the cure, that is, before the molecules are all cross-linked. This reaction occurs rapidly even at ambient temperatures. The resins become hard and brittle and thereafter little time is available in which to work with the resins before they set up. It is desirable, therefore, to have a form-ulation which results in tough chemically resistant products yet would have a long pot life, i.e., B-stages very slowly, allowing greater working time in regard to handling of the resin. Such long pot life is very important in applications in which a premix of the resin and other materials such as fibrous reinforcing materials is utilized.
It is known that imidizoles are relatively fast curing agents for epoxy resins. Such materials lack stability in mixutres with epoxy resins at room temperature for long periods of time and thus are usually not suitable as curing agents in formulations requiring extended pot life. Some imidazoles have been used to accelerate the reaction between epoxy resins and aromatic amines. Such imidazoles, their salts, and their uses with respect to epoxys are set forth in U.S. Pat. Nos. 3,533,985; 3,553,166; 3,493,630; 3,755,253; 3,409,592; 3,746,686 and 3,642,698.